feat: implement hash to G2 on BLS12-381

[ivokub]

Description

This PR takes #1447 and #1040 and Consensys/gnark-crypto#674, implementing Map/Hash to G2 over BLS12-381.

It is still work in progress, i'm refactoring to make it compatible with the non-native group algebra we have elsewhere. I'll also generify mapping bytes to non-native elements more efficiently (instead of doing it byte-by-byte).

Type of change

• New feature (non-breaking change which adds functionality)

How has this been tested?

• Test A
• Test B

How has this been benchmarked?

• Benchmark A, on Macbook pro M1, 32GB RAM
• Benchmark B, on x86 Intel xxx, 16GB RAM

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules